October 8, 2013

MIT Researchers Create Their Own Martian Clouds Right Here On Earth

The clouds on Mars, at first glance, might be easily mistaken for those on Earth. NASA's Opportunity rover returned images of the Martian sky with gauzy, high-altitude wisps that are similar to our cirrus clouds. These clouds most likely consist of either carbon dioxide or water-based ice crystals, given what scientists currently understand about the Martian atmosphere. However, scientists say without sampling the Martian clouds directly, it is difficult to know the precise conditions that give rise to such clouds.

A research team from MIT has done the next-best thing. The team has turned to a three-story-tall cloud chamber in Germany to recreate Mars-like conditions. They can adjust the chamber's temperature and relative humidity to match conditions on Mars — essentially forming Martian clouds on Earth.

The research team was able to create clouds at the frigid temperatures typically found on Mars. However, they found that cloud formation in such conditions required adjusting the chamber's relative humidity to 190 percent. This is far greater than required for cloud formation on Earth. These results should help to improve future conventional models of the Martian atmosphere, the majority of which assume that Martian clouds require humidity levels comparable to those on Earth for cloud formation.

"A lot of atmospheric models for Mars are very simple," says Dan Cziczo, the Victor P. Starr Associate Professor of Atmospheric Chemistry at MIT. "They have to make gross assumptions about how clouds form: As soon as it hits 100 percent humidity, boom, you get a cloud to form. But we found you need more to kick-start the process."

The researchers' experimental results will help to improve Martian climate models, according to Cziczo. This will also improve the understanding of how the planet transports water through the atmosphere. The findings of this study were published in Journal of Geophysical Research: Planets.

The majority of the experiments were conducted during the summer of 2012 in Karlsruhe, Germany, at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility. This facility is a former nuclear reactor that has been converted to the world's largest cloud chamber.

Originally, the facility was designed to study atmospheric conditions on Earth. Cziczo, however, realized that with a little tweaking, the chamber could be adapted to simulate atmospheric conditions on Mars. First, all of the oxygen in the chamber was replaced with inert nitrogen or carbon dioxide - the most common components of the Martian atmosphere. Then, the researchers created a dust storm by pumping in fine particles similar in size and composition to the mineral dust found on Mars. These particles act as cloud seeds, with water vapor adhering to them to form cloud particles.

After the chamber was seeded, the team adjusted the temperature. First, they set it to the coldest temperatures at which clouds form on Earth (around minus 81 degrees Fahrenheit), then moved it progressively lower. Eventually, they stopped at the chamber's lowest setting, around minus 120 Fahrenheit — "a warm summer's day on Mars," Cziczo says.

Adjusting the chamber's relative humidity allowed the researchers to create clouds under warmer, Earth-like temperatures, at expected relative humidities. The researchers gained confidence in their experimental setup from these observations as they attempted to grow clouds at temperatures that approached Mars-like conditions.

The team created 10 clouds over a week's time, with each cloud taking about 15 minutes to form. Because the chamber is completely insulated, they used a system of lasers, which beam across the chamber, to detect cloud formation. Clouds that form scatter the laser light, which is detected and recorded by computers that display the results - including size, number, and composition of cloud particles.

The research team analyzed these results for the next six months to find that clouds that grew at the lowest temperatures required extremely high relative humidity in order for water vapor to form an ice crystal around a dust particle. Why Martian clouds need such humid conditions is still unclear, but Cziczo hopes to investigate the question further.

The research team intends to return to Germany next fall. By that time, the chamber will have undergone renovations, allowing it to perform cloud experiments at even lower temperatures — conditions that may more closely mimic the icy atmosphere on Mars.

"If we want to understand where water goes and how it's transported through the atmosphere on Mars, we have to understand cloud formation for that planet," Cziczo says. "Hopefully this will move us toward the right direction."